More Is Better

More loudspeakers really can make better quality sound. Distributed loudspeakers are one example of the old maxim -- more is better. The intention of most distributed loudspeaker systems, from the lowly paging system to the outdoor music festival sound reinforcement system, is to provide the same sound quality over a large area. This democratic ideal is most often served by placing loudspeakers within 6 dB of one another. A simple criterion that is often misunderstood.

While the dispersion and output level of paging loudspeakers has very little in common with the large concert array, the inverse-square law applies equally to all sound sources (except the most coherent of line arrays and few of them can achieve consistent coupling across the audio spectrum). In essence, no matter what the dispersion of the loudspeaker, distance plays an important part in the determination of how many loudspeakers are needed to provide the intended listening area with even sound levels. Typically, the loudspeakers will be spaced to ensure that each listener is provided with the same volume, +/- 3 dB. This allows an acceptable range of variation that can be met in practical installations.

For the sake of simplicity, I will use the ceiling loudspeaker system as an example of how distance from a sound source influences the change in volume across the intended listening area. Ceiling loudspeakers will often be specified to have a 90-degree dispersion over the frequency band suited to the application. Wider coverage angles may be selected to achieve high-frequency coverage suited to music, but can do little to reduce the spacing between the loudspeakers. This is because; even extremely wide angles of coverage cannot change the spreading of the wavefront that forms the basis for the inverse-square law. As the sound wavefront moves away from the source, the spreading of the energy over a wider area reduces the energy at any one point within the wavefront. The sound has half the pressure when observed (auditioned) at twice the distance from the source. This leads us to the 6 dB loss in SPL (sound pressure level) with each doubling of distance.

Let's return to our example of the ceiling loudspeaker. If the device is mounted in a room with an eight-foot ceiling height and the audience is seated; the nearest listener (directly beneath the loudspeaker) is four feet away from the loudspeaker. The listener seated at 90-degrees from the loudspeaker is now 8.5 feet away, or roughly double the distance from the sound source. Therefore, even if the loudspeaker can achieve 180-degree coverage across the desired frequency spectrum, listeners beyond 45-degrees will not be within the +/- 3 dB nominal coverage criterion. In an eight-foot ceiling, the loudspeakers must be spaced no more than 12 feet apart. With this spacing, the combined energy from the adjacent loudspeakers will make up for the roll-off in level at the edge of their coverage pattern, in theory.

In fact, the loudspeakers must be spaced to achieve suitable performance for the application. If music quality high-frequency response is required and the +/- 3 dB criteria for both broadband level and frequency response is to be met, then the spacing will be closer together. If the application merely requires the sound to be audible (+/- 10 dB) throughout the room, then the spacing between loudspeakers can be much greater. In either case, it is the allowable limits of the level variation between seating locations that will determine the maximum distance between the loudspeakers, and not simply the coverage angle indicated in the manufacturer's specifications.

There have been horn technologies, such as the Altec-Lansing Vari-Intense horn, that can overcome the inverse-square law restrictions. High-directivity horns can also be combined into arrays to reach beyond double the distance between the nearest and furthest listeners. While this is relatively straightforward at mid-band frequencies, it can be far more complicated at low and high frequencies. Inevitably, the system will require more loudspeakers, if uniform level is required over a deep audience. The geometric limitations of the inverse-square law will force low-directivity loudspeakers to be located higher up above the audience, to maintain uniformity in level. Where height is not an option, then the quantity of loudspeakers will usually increase. Whether the loudspeakers are delay stacks for an outdoor concert or rows of ceiling loudspeakers in a convention centre ballroom, the same basic rules apply.

The next time someone tells you that their loudspeaker can fill an entire venue from a single place, ask them how they have overcome the limitations of the inverse-square law. If their eyes glaze over, it may be time to find someone that has given the issue a little more thought. They may sell you more loudspeakers, but the end result is likely to sound better. More is better than +/- 3 dB.

United Entertainment Media Inc.
460 Park Avenue South, 9th Floor
New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

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